Segmentation fault (core dump) when runing more events

Hi Dear GEANT4 users;
I try to simulate the total energy deposited by volumetric gamma source in the water geometry, located in the center of my source. I modified the example B1, with using GPS as my primary generator. All work fine when running event less than 2000. Increasing more events, the running stopping around 3000 and I had been the following message: Segmentation fault (core dumped).
Anyone can help ?